1. Field of the Invention
The present invention relates to an apparatus and method for monitoring wavelength division multiplexed optical signals including multiple channel optical signals in a wavelength division multiplexing (WDM) system.
2. Description of the Related Art
In WDM systems, in which multiple optical channels are multiplexed and carried over a single optical fiber, channel quality monitoring is usually carried out after demultiplexing of each channel. However, recently, there is an increasing need for monitoring optical signal quality in a WDM state prior to the monitoring of the channel quality.
For monitoring multiple channel optical signals, the measurement on the optical power, the optical wavelength and the optical signal to noise ratio (OSNR) for each optical channel signal is required. In a WDM optical transmission system, the transmissions of the signals of neighboring channels as well as of the signal of the current channel are susceptible to error, and thus exact control of wavelength deviation for each channel is required. Also, there is a need to monitor the control of wavelength in real time.
In the use of a WDM optical amplifier, the optical power and the OSNR may vary for each channel. Thus, the channel-based monitoring thereon is required. Usually, the overall signal quality can be inferred from the OSNR and the optical power of each channel, and thus the factors of the OSNR and the optical power are necessarily measured to monitor the signal quality.
U.S. Pat. No. 5,617,234 to Masafumi Koga et al. entitled Multiwavelength Simultaneous Monitoring Circuit Employing Arrayed-Waveguide Grating, and incorporated by reference herein, teaches a multiwavelength simultaneous monitoring circuit employing an arrayed waveguide grating (AWG) and a photo diode array, which is capable of monitoring the wavelength for each channel. However, the disclosure requires that the AWG temperature be controlled in real time, additionally needs an optical signal for a reference wavelength, and fails to achieve the correct measurement of the optical power and the OSNR. In the channel monitoring of the WDM optical transmission system, the optical power, the wavelength, and the OSNR must be simultaneously monitored. However, the disclosure permits the measurement of only two of the three factors. That is, if two arbitrary factors are measured, it is not possible to measure the remaining one factor.
Of interest to the present invention, and incorporated by reference herein, are U.S. Pat. No. 5,894,362 to Hiroshi Onaka et al. entitled Optical Communication System Which Determines The Spectrum Of A Wavelength Division Multiplexed Signal And Performs Various Processes In Accordance With The Determined Spectrum; U.S. Pat. No. 5,850,292 to David M. Braun entitled Wavelength Monitor For Optical Signals; and U.S. Pat. No. 5,915,052 to Gary A. Ball entitled Loop Status Monitor For Determining The Amplitude Of The Signal components Of A Multiwavelength Optical Beam.
It is an object of the present invention to provide in a wavelength division multiplexing (WDM) system an apparatus and method for monitoring the quality of optical channel signals by measuring the optical power and the wavelength thereof.
It is another object of the present invention to provide in a wavelength division multiplexing (WDM) system an apparatus and method for monitoring the quality of optical channel signals by simultaneously measuring the optical power, the wavelength and the optical signal to noise ratio (OSNR) thereof.
According to an aspect of the present invention, there is provided an optical signal monitoring apparatus comprising: an optical demultiplexer, having 2N output ports, for receiving and demultiplexing, by wavelength, a wavelength division multiplexing (WDM) signal with N wavelengths, and outputting two output port signals for each channel via the 2N output ports; a signal conversion unit for receiving the output port signals from the optical demultiplexer and converting the received optical output port signals into digital signals; and a signal processing unit for receiving the digital signals and calculating the wavelength and optical power for each channel using the loss characteristics of the two output port signals of each channel.
According to another aspect of the present invention, there is provided a method of monitoring an optical signal, comprising the steps of: (a) demultiplexing by wavelength a wavelength division multiplexing (WDM) signal with N wavelengths into two optical signals for each channel, to output a total of 2N optical signals; (b) converting the optical signals from the step (a) into digital signals; and (c) receiving the digital signals to calculate the wavelength and the optical power for each channel, using the loss characteristics of the two optical signals of the corresponding channel.